This invention relates to an electric power steering unit to be used for a vehicle and, more particularly, it relates to a field of technology effectively applicable to electric power steering gears to be used for rack and pinion type steering systems.
For assisting steering power of vehicles, a so-called power steering unit is equipped to many vehicles in recent years, and various power steering units such as hydraulic type, electric type, etc. are proposed. Of these power steering units, for the electric type power steering unit applied to rack and pinion type steering units (so-called electric power steering), the one called rack-assist type that obtains steering assisting power by a motor equipped close to the rack shaft is known.
FIG. 5 is an explanatory view showing one example of this kind of rack-assist type power steering unit. As shown in FIG. 5, the electric power steering unit has a motor 52 installed coaxially with a rack shaft 51, and the steering assisting power generated by the motor 52 is transmitted to the rack shaft 51 via a ball screw mechanism 53. And with this steering assisting power and manual steering power, guiding wheels are turned to alleviate steering loads of a driver.
The rack-shaft 51 is linked to the guiding wheels typically by way of tie rods or knuckle arms arranged at the respective opposite ends thereof and also linked to the steering column 54 that is coupled to the steering wheel by way of a rack and pinion gear so that it may be reciprocatively moved in the horizontal directions of FIG. 5 as the steering operation by the driver. The motor 52 is arranged in a housing 71 containing coupling section 68 between the steering column 54 and the rack shaft 51, and a cylindrical armature shaft 56 and a field device 57 coaxially inserted in a cylindrical yoke 55. In the power steering unit of FIG. 5, the yoke 55 is formed integral in the housing 71. In addition, the field device 57 comprises a magnet 59 installed to the inner circumferential portion of the yoke 55 and an armature core 60 installed to the outer circumferential portion of the armature shaft 56. And electric power is supplied to this motor 52 via a power supply section 58 equipped with a brush 61 and a commutator 76 in slidably contact therewith.
On the other hand, the ball screw mechanism 53 is of a generally known construction in which a large number of balls 64 are intermediately inserted between a nut section 62 and a screw section 63. In this case, the nut section 62 is held rotatably in the housing 72 by an angular bearing 65 in the condition housed in a holder 75. The rotating force of the motor 52 is transmitted from the armature shaft 56 to the rack shaft 51 via the nut section 62, balls 64, and screw section 63 in the form of axial reciprocating motion, and assists the steering power.
Now, in the power steering unit of FIG. 5 comprising in this way, both housings 71 and 72 are assembled by directly coupling the housings with screws. In a conventional power steering unit, right and left housings have a flange protrudably formed at each end portion, respectively, and by connecting these with bolts, the housings are coupled. However, since the power steering unit is generally installed near the road surface of the car body and must be housed in a limited space below the engine room, it is not desirable to have protrusions on the outer circumference of the device and the improvements have been waited.
Therefore, in the power steering unit of FIG. 5, a system for coupling the housings with screws as shown in FIG. 6 is adopted, and here, the device configuration free of protrusions with the flange removed has been achieved. FIG. 6 is an enlarged sectional view of a main portion showing the configuration of the coupling portion. In the figure, a female threaded portion 73 is formed on the left inner circumference of the housing 71, while a male threaded portion 74 is formed on the right end outer circumference of the housing 72. And by driving a male threaded portion 74 into this female threaded portion 73, both housings 71 and 72 are coupled together.
However, in the power steering unit of FIG. 5, since both housings 71 and 72 are formed with same material by aluminum die casting, etc., there is a problem in that the contact surface wears and is easy to adhere by surface pressure and friction caused by tightening. Consequently, lubrication and surface treatment are required between both housings, also resulting in a problem of disadvantages from the viewpoint of man-hour or parts cost.
In addition, because both housings 71 and 72 are formed with nonferrous metals such as aluminum, etc. for weight reduction, there was a problem of decreased coupling strength or rigidity at the threaded portion. Consequently, as shown in FIG. 6, the wall thickness at the threaded portion must be increased to secure the strength of the coupling portion, also producing a problem of going against needs of reduced weight, reduced thickness, and reduced size.
It is an object of the present invention to prevent wear and adhesion and to reduce the wall thickness with the coupling strength of housings secured in an electric power steering unit of a configuration in which housings are coupled with screws.
The above-described and other objects and novel feature of the present invention will become apparent more fully from the description of the following specification in conjunction with the accompanying drawings.
The electric power steering unit according to the present invention comprise a rack- shaft to be coupled to the guiding wheels of a vehicle, a first housing for movably containing the rack-shaft and holding therein a coupling section for causing the teeth of the rack-shaft to engage those of a pinion arranged on the steering column linked to the steering handle of the vehicle, an electric motor equipped with a yoke integrally formed with the first housing and coaxially arranged around the rack-shaft to supply assisting power to the rack-shaft, a ball screw mechanism for connecting a nut section linked to the armature shaft of the electric motor with a screw section formed on the rack-shaft by disposing ball members therebetween and for transmitting the rotary power of the electric motor to the rack-shaft as assisting power for steering, and a second housing for containing the ball screw mechanism, characterized in that a first threaded portion is formed at the end portion of the yoke on the second housing side, a second threaded portion is formed to fit to the first threaded portion on the end portion of the second housing on the first housing side; and the first housing and the second housing are coupled together by fitting the first threaded portion to the second threaded portion.
With this configuration, the first threaded portion and the second threaded portion fitted to it are able to be formed with different materials, and it becomes possible to prevent wear or adhesion on the contact surface caused by fitting screws of the same material.
On the other hand, the first threaded portion may be formed into female screw and the second threaded portion into male screw, and in such case, it is possible to form the female screw on the steel-made yoke, increasing the coupling strength and rigidity at the threaded portion, and this no longer requires to form the coupling portion to have large wall thickness, and the housing is able to reduce wall thickness.
In addition, the first threaded portion may be made into a male screw and the second threaded portion into a female screw. Preferably, the first threaded portion may be made of steel, and the second threaded portion may be made of aluminum.
Furthermore, caulking grooves may be formed on the outer circumference of the second housing, and the end portion of the first housing on the second housing is caulked to these caulking groove, thereby forming caulking portions to fix the first and the second housings. Preferably, the caulking grooves may be formed continuously throughout the whole circumference of the second housing or a plurality of caulking grooves may be intermittently formed on the outer circumference of the second housing.